Activated carbon is widely used in the food industry, chemical industry, pharmaceutical industry and various other industries; in concrete terms, examples of use include applications such as clean water manufacturing applications, air cleaning applications, solvent recovery applications, stack effluent desulfurization and denitrogenization applications, decoloring applications, tap water treatment, sewage treatment, treatment of feces and urine, industrial waste water treatment, sugar refining applications, nuclear power applications (adsorption of radioactive substances), methane occlusion, hydrogen occlusion and the like. These applications mainly utilize the adsorption performance of activated carbon. But, the activated carbon as an adsorbing agent that is more superior in terms of adsorption performance is desired.
In recent years, meanwhile, electrical double layer capacitors have attracted attention as back-up power supplies, auxiliary power supplies and the like, and development focusing on the performance of activated carbon as electrodes used in electrical double layer capacitors has been widely undertaken. Electrical double layer capacitors which use activated carbon in polarizing electrodes are superior in terms of electrostatic capacitance; accordingly, along with development in the field of electronics, there has been a rapidly growing demand in electronic device electrode applications and the like. Recently, furthermore, in addition to miniaturization in conventional memory back-up power supplies and the like, there has also been development of high-capacitance products used in the auxiliary power supplies of motors and the like.
Among these fields of utilization of activated carbon, activated carbon that contains no alkali metals or heavy metals is preferable in the fields of food products, drug manufacture, clean water and electronic devices. Conventionally, therefore, methods for manufacturing activated carbon have generally been methods in which a carbonaceous material is subjected to a gas activation treatment or a chemical activation treatment, e.g., an alkali activation treatment using an alkali metal hydroxide as an activation assistant, and the system is then washed with a strong acid such as hydrochloric acid, nitric acid, sulfuric acid or the like in order to remove alkali metals and heavy metals from the product of the activation treatment.
However, in cases where activated carbon washed with a strong acid is used as a raw material in electronic devices, e.g., an electrode material in non-aqueous electrolyte secondary batteries or electrical double layer capacitors, problems such as shorting and the like occur as a result of dendrite formation caused by the reductive deposition of alkali metals and heavy metals; furthermore, self-discharging tends to occur as a result of alkali metal ions and heavy metal ions, so that the problem of a low rate of electrostatic capacitance retention due to self-discharging is encountered (below, the rate of electrostatic capacitance retention due to self-discharging will be abbreviated to “the rate of self-discharge retention”).
Especially in the case of an alkali activation treatment using an alkali metal hydroxide as an activation assistant, since the alkali metal hydroxide is strongly oxidizing compound, corrosion of the heating furnace used for activation occurs during the activation treatment, and heavy metals are admixed with the product of the activation treatment, so that even if this product of the activation treatment is washed with hydrochloric acid or nitric acid, it is extremely difficult to manufacture activated carbon from which heavy metals have been completely eliminated. If activated carbon containing admixed heavy metals is used as a raw material in electronic devices, e.g., as a raw material for polarizing electrodes in an electrical double layer capacitor, heavy metal particles formed into dendrites will be formed on the separators of such electrical double layer capacitors by the reductive deposition of heavy metals as described above, so that problems such as the opening of holes in the separators or the like occur, thus leading to trouble such as short-circuiting or the like. Furthermore, there may be cases in which alkali metals originating in the alkali metal hydroxides used as activation reagents remain in the activated carbon. If such activated carbon is used as a raw material for polarizing electrodes in electrical double layer capacitors, the leakage current is increased, so that the charging efficiency drops, thus resulting in a poor energy efficiency (in other words, the rate of self-discharge retention drops).
For example, an electrical double layer capacitor using activated carbon with an Fe content of 200 ppm or less, a Cr content of 10 ppm or less, an Ni content of 10 ppm or less, an Na content of 200 ppm or less, a Cl content of 300 ppm or less and an ash content of 0.5% or less as a polarizing electrode material has been proposed in Japanese Patent Application Laid-Open No. 1-241811. In this publication, it is indicated that metal components are admixed in the manufacturing process of the activated carbon, and that the elution of these metal components causes a drop in the long-term reliability of the electrical double layer capacitor. However, there is no description of how to suppress such metal contents in order to realize a reliable electrical double layer capacitor, although it is described that the elution of these metal components causes a drop in the long-term reliability of the electrical double layer capacitor.
Furthermore, activated carbon for use in the polarizing electrodes of electrical double layer capacitors which allows the use of an alkali activation process, and which is manufactured by washing with water, an acid solution and then an alkali solution following activation, is disclosed in Japanese Patent Application Laid-Open No. 2002-43190, and it is indicated that self-discharge can be reduced by reducing the Ni content. However, an electrical double layer capacitor with desired performance cannot be constructed merely by reducing the Ni content.
Accordingly, it is an object of the present invention to provide activated carbon which does not lead to dendrite formation by the reductive deposition of alkali metals or heavy metals when used as a raw material in electronic devices, so that problems such as short-circuiting tend not to occur, and which shows a high rate of self-discharge retention, so that this activated carbon is suitable for use in applications such as electronic devices and the like.